Re: [Haskell-cafe] Type Family Relations

Hi, I like collecting examples of different type system related issues, and I am curious in what way is the solution that I posted limited. Do you happen to have an example? Thanks, Iavor On Sat, Jan 3, 2009 at 8:35 PM, Thomas DuBuisson wrote:

Thank you all for the responses. I find the solution that omits type families [Diatchki] to be too limiting while the solution 'class (Dual (Dual s) ~ s) =>' [Ingram] isn't globally enforced. I've yet to closely study your first solution, Ryan, but it appears to be what I was looking for - I'll give it a try in the coming week.

Tom

On Sat, Jan 3, 2009 at 8:18 PM, Iavor Diatchki wrote:

...

Hello, Usually, you can program such things by using super-classes. Here is how you could encode your example:

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances #-}

class HeaderOf addr hdr | addr -> hdr class HeaderOf addr hdr => AddressOf hdr addr | addr -> hdr

data IPv4Header = C1 data IPv4 = C2 data AppAddress = C3 data AppHeader = C4

instance AddressOf IPv4Header IPv4 instance HeaderOf IPv4 IPv4Header

{- results in error: instance AddressOf AppHeader AppAddress instance HeaderOf AppAddress [AppHeader] -}

Hope that this helps, Iavor

On Sat, Jan 3, 2009 at 7:22 AM, Thomas DuBuisson wrote:

...

Cafe, I am wondering if there is a way to enforce compile time checking of an axiom relating two separate type families.

Mandatory contrived example:

...

type family AddressOf h type family HeaderOf a

-- I'm looking for something to the effect of: type axiom HeaderOf (AddressOf x) ~ x

-- Valid: type instance AddressOf IPv4Header = IPv4 type instance HeaderOf IPv4 = IPv4Header

-- Invalid type instance AddressOf AppHeader = AppAddress type instance HeaderOf AppAddress = [AppHeader]

So this is a universally enforced type equivalence. The stipulation could be arbitrarily complex, checked at compile time, and must hold for all instances of either type family.

Am I making this too hard? Is there already a solution I'm missing?

Cheers, Tom _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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